1. Field of the Invention
The present invention relates to a method of manufacturing a semiconductor device and, more particularly, to a method of manufacturing a semiconductor device having a capacitor, such as a DRAM.
2. Description of the Prior Art
A capacitor insulating film of a DRAM has a two-layered structure constituted by a silicon oxide film and a silicon nitride film. This capacitor insulating film is obtained as follows. A silicon nitride film is grown on a polysilicon film as a lower electrode by a low-pressure chemical vapor deposition method, and then a silicon oxide film is formed on the surface of the silicon nitride film by wet oxidation.
As shown in FIG. 1, however, a native oxide film 8 is present on the interface of a silicon nitride film 9 and a polysilicon film serving as a lower electrode 7. The presence of this native oxide film decreases the capacitance, and increases the current leakage from a capacitor insulating film, resulting in degradation of dielectric breakdown characteristics. For this reason, in order to completely remove the native oxide film on the interface, the native oxide film is removed, and the silicon nitride film is immediately formed on the polysilicon film in, e.g., the same reaction furnace without exposing the surface of the polysilicon film to an oxygen atmosphere. This is effective in decrease current leakage and improving dielectric breakdown resistance.
As a conventional technique of removing a native oxide film on silicon, a method of performing heating in a H.sub.2 atmosphere, and reducing a native oxide film with hydrogen to remove the film, and a method of performing heating in a reactive gas (e.g., 1% HCl-99% H.sub.2), and etching a native oxide film together with the surface of a substrate to remove the film are considered as preprocessing before forming a gate oxide film, unlike preprocessing before forming a capacitor insulating film (both the methods are disclosed in Japanese Unexamined Patent Publication No. 2-150029). Of the two methods, the method of performing heating in the H.sub.2 gas atmosphere, and reducing a native oxide film with hydrogen to remove the film will be described below.
According to this method, a silicon substrate is prepared as a substrate, and the silicon substrate is cleaned before oxidation using normal chemicals, distilled water, and the like. In order to prevent the growth of a native oxide film, the silicon substrate is set in a reaction furnace in a N.sub.2 atmosphere. After the reaction furnace is evacuated into a high vacuum, H.sub.2 is introduced into the reaction furnace. While introducing H.sub.2, a reduced pressure state at a low vacuum of 1.3 to 1.3.times.10.sup.4 Pa is kept, and heating of the silicon substrate is controlled to keep the silicon substrate at a surface temperature of 1,000.degree. C. for about 10 to 30 seconds. The heating of the silicon substrate is stopped, and the supply of H.sub.2 is also stopped. After the silicon substrate is cooled to room temperature, the reaction furnace is evacuated to a high vacuum. In order to form a silicon oxide film on the silicon substrate, O.sub.2 is introduced into the reaction furnace. Heating of the substrate is adjusted to keep the substrate at a temperature of about 1,100.degree. C. for about 20 seconds. The heating of the silicon substrate is stopped, and the supply of O.sub.2 is also stopped. Then, the substrate is cooled to room temperature. According to the above method, the silicon oxide film is formed on the surface of the silicon substrate after removing the native oxide film.
As preprocessing for growing a GaAs layer on a silicon substrate, a technique of removing a native oxide film by which heating is performed in a GeH.sub.4 atmosphere is known (Japanese Unexamined Patent Publication No. 3-270236).
According to this method, a silicon substrate is set in an organic metal chemical vapor deposition furnace. GeH.sub.4 is introduced into the reaction furnace at 1.times.10.sup.4 Pa, and the silicon substrate is heated to 200.degree. to 400.degree. C. to anneal. By this processing, a native oxide film on the silicon substrate is removed to make a clean silicon surface appear. Next, trimethylgallium and AsH.sub.3 are introduced into the reaction furnace, and a GaAs layer is grown on the silicon substrate under conditions at a growth temperature of 700.degree. C., and a pressure of 1.times.10.sup.4 Pa.
When the above technique of removing a native oxide film is used before formation of a silicon nitride film for a capacitor insulating film, the following problems ensue.
(1) According to the method of removing a native oxide film by heating in a hydric atmosphere, a heating temperature of about 1,000.degree. C. is required. Such a temperature has no problem as preprocessing before formation of a gate oxide film, as a stage before forming a source/drain region of a MOS transistor. However, when the method of removing a native oxide film by heating in a hydric atmosphere in the H.sub.2 atmosphere is applied to preprocessing before formation of a capacitor insulating film of a stacked capacitor where a MOS transistor has been formed, heating at about 1,000.degree. C. causes an increase in width of the source/drain diffusion layer, and this makes it impossible to form a fine element.
(2) According to the method of removing a native oxide film by heating in a reactive gas (e.g., 1% HCl-99% H.sub.2) atmosphere, since silicon is undesirably etched at the same time of removing a native oxide film, the surface of a low electrode is roughened to increase the current leakage from a capacitor.
(3) According to the method of removing a native oxide film by heating in a GeH.sub.4 atmosphere, a native oxide film can be removed at a relative low heating temperature of about 200.degree. to 400.degree. C. GeH.sub.4 used in this method, however, easily self-decomposes by external factors such as vibrations, and its pressure increases to about 20 times, resulting in difficult handling in terms of safety. When this method of removing an oxide film is applied to preprocessing before formation of a capacitor insulating film, the step of completely substituting an atmosphere is required between the step of removing the native oxide film using GeH.sub.4 and a step of forming a silicon nitride film.